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Last month, the FDA set the safety threshold for melamine at 2,500 parts per billion for foods other than infant formula. The agency said it did not have enough data to set a safety threshold for infants.

A spokeswoman for the Atlanta, Georgia-based International Formula Council, a trade group, said she had not seen the data, but was encouraged that the quantity found was below levels deemed safe in infants by the governments of China, Malaysia, the Philippines, Hong Kong, Canada and New Zealand.

"Apparently these trace levels can be found in lots of food," said spokeswoman Mardi Mountford.

The FDA also found trace levels of melamine in several samples of medical formula supplements for the elderly, but the amounts posed no health risk to adults, Leon said.

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Though U.S.-based companies can import ingredients from China for nutritional supplements for adults, there is an import ban on dairy products and components, she said.

Melamine is an industrial chemical used in the manufacture of can liners, flame retardant, cleaning products, fertilizers and pesticides.

It does not occur naturally in food.

Because it contains nitrogen, its addition to food products can wrongly suggest an inflated protein content. Ingesting melamine in large doses over an extended period of time could cause kidney stones and other illnesses, though small amounts pose no such danger for adults, agriculture and health experts say.

Its presence in Chinese infant formula has led to the hospitalization of more than 12,000 children and the deaths of several in China, according to the FDA, which said it is not aware of any such illnesses in the United States.

This month, the FDA announced that it was limiting the import of all dairy products from China until they have been proven free of melamine.